toSentenceCase method
Converts the string to sentence case (e.g., "hello world" -> "Hello world").
Capitalizes the first letter of the string and keeps the rest unchanged.
Example:
"hello world".toSentenceCase(); // Returns "Hello world"
"".toSentenceCase(); // Returns ""
Implementation
String toSentenceCase() {
if (isEmpty) return this;
final trimmed = trim();
return trimmed[0].toUpperCase() + trimmed.substring(1);
}